Properties of powder coatings in load carrying construction.

The present study describes the mechanical behavior of powder coatings used under very high compressive loads in clamping force joints.

Carboxyl functional polyester powder coatings cured with hydroxyl functional [beta]-hydroxyalkylamides, with variations in coating thickness and amount and type of filler, have been studied. The coatings were subjected to conventional tests for coatings and polymers and also to specially designed tests developed to study the behavior of powder coatings in clamping force joints. The specially designed tests were used to study the coatings under compressive loads, and the relation between the results from these test methods and from conventional tests is discussed.

The results show the importance of coating thickness in order to achieve the desired mechanical properties of a coating when used under high compressive loads. These loads put high demands on the stability of the coating, and the defects must be kept to a minimum. Increased thickness will give rise to more defects in the coating, especially voids and blisters due to the evaporation of water formed during the curing of the polyester powder coating. The surface roughness of the coating is also affected by the coating thickness, but the main influence originates from the type and amount of filler used. A rough surface will give rise to stress concentrations and increased plastic deformations in the coating, impairing the properties of the clamping force joint.

Keywords: Dynamic mechanical properties, polyesters, powder, mechanical properties, automotive OEM, CPVC, iron, fillers, cast iron, steel

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Powder coatings find an increased use in the coating industry and often replace conventional wet paints because of their more environmentally friendly production and application. (1-6) Powder coatings also show superior mechanical behavior such as stone chip resistance and resistance to scratch and mar in many cases. Powder coatings are normally cured at high temperatures in order to obtain a good resin flow, an even film, and rapid crosslinking of the coating. The high curing temperature reduces problems with vitrification and allows a high glass transition temperature of the cured film to be obtained. The glass transition temperature, [T.sub.g], is often in the range of 60-80[degrees]C, which means that the coating is normally used below the [T.sub.g]. (7-9)

The properties of the coating depend on numerous factors such as chemical structure, crosslink density, filler and pigment content, curing conditions, curing mechanism, etc. (10,11) The properties also often vary throughout the layer due to curing conditions and substrate type. The substrate may, for example, act as a heat sink forming a temperature gradient through the film during oven curing. This temperature gradient may induce a property gradient in the final film. (12) A good adhesion to the substrate also restricts the mobility of the coating at this interface compared to the air-interface where no restriction exists.

The increased use of precoated materials in various assemblies has intensified the demand for a better understanding of the overall mechanical performance of coated parts. One example where this is evident is the use of powder coated steel parts in the automotive industry. Moreover, a coating is often subjected in different degrees to compressive, tensile, and shearing deformations. Therefore, the load conditions have to be carefully defined in each case. Coated steel objects are often used in constructions subjected to compressive loads such as screwed or bolted joints. The long-term mechanical stability of the coating is of crucial importance in these constructions. The properties of the whole construction can be severely affected if the coating film deforms during use. In a clamping force joint, as shown in Figure 1A, the clamping force is dependent on the elastic tension of the screw and the compressive strain of the steel parts mounted in contact (Figure 2). The situation is still more complicated when coated steel objects are used in the joint, as in Figure 1B. The steel substrate has a much higher elastic modulus and yield strength compared to the polymer/coating, and the steel coated with a thin organic film may be regarded as a two-level composite part. The coating film itself is a composite material on a smaller scale consisting of a particle-reinforced polymer.

[FIGURE 1 OMITTED]

The clamping force joint is described in more detail in the experimental section.

Studies on clamping force joints constructed of composites, and the effect of the properties of the polymer on the loss of clamping force, have been presented by Sullivan et al. (13) For example, it was observed that the properties of the material did not behave in a similar way under compressive load as when subjected to tensile load.

Our knowledge of the mechanical properties of polymers is often based on tensile tests, but it is not always fully understood how the structure of the polymer affects the mechanical properties under compressive load. In the case of a coating on a rigid substrate, the behavior is even more difficult to describe. The effect of the substrate on the mechanical properties of the coating is known to be significant. (14) It has been shown that the mechanical properties and the composition of the coating do affect the function of a clamping force joint made of coated steel objects. (15) However, the knowledge of how the coating properties can be tailored for the use under compressive loads is still to be developed. A better understanding of how coatings behave under compressive loads will make it possible to predict the performance of a specific coating material and the design of the joint.

A previous study on highly crosslinked thermosets subjected to high compressive loads indicated a strong influence from the geometry on the behavior of the compressed part. (16) A polymer subjected to loads will deform in a viscoelastic, time-dependent manner. This can be clearly seen in both tensile and compressive tests when large samples are used. However, when the thickness of the material is decreased, the deformation is more and more restricted by the surrounding material and by friction between samples and friction between samples and equipment. A thin sample has a much lower creep strain rate during constant load but also shows a higher apparent modulus due to the restricted possibility of the material deforming. The Poisson's ratio of a crosslinked polymer is much higher than that of a metal, and, thus, the modulus of a constrained polymer subjected to a unidirectional load will appear higher than in unconstrained mode. (17) A contribution of the substrate properties to the overall behavior has been observed for thicknesses up to 5 mm. This is far more than conventional coating thicknesses, so the properties of the coating and substrate in combination are of definite interest for the compressive behavior of a coated steel object.

[FIGURE 2 OMITTED]

The final properties of a clamping force joint are not only dependent on the compression strength of the coatings and objects, but also on the friction of the different parts in the joint and how they interact during mounting and during use. The friction between coating and nut is of importance when mounting the joint. During mounting a torque is applied to the screw and nut. The part of the torque transferred into clamping force is dependent on the friction in the set-up. The torque applied to the screw and nut will result in a high clamping force or even plasticization of the screw, if the friction is too low during mounting. On the other hand, if the friction is too high the applied torque will only be used to overcome the friction, and the clamping force will be too low. Therefore, the friction of the coating during mounting is of importance for the properties of the joint.

In this study, carboxyl functional polyester powder coatings of various physical structures have been tested. Both conventional coating tests such as stone ship resistance and adhesion tests have been used, but also specially designed tests in order to simulate the strains and stresses of a coating in a clamping force joint. The aim of this study is to find a correlation between the physical structure of the coating and its behavior under compressive loads in a clamping force joint. A coating can be seen as a thin polymer composite consisting of a thermoset resin reinforced with inorganic fillers. The types and amounts of fillers are usually varied in order to achieve desired properties regarding scratch and mar resistance. (5,6) This study focuses on these properties under compressive loads and how the fillers affect the performance of the coating.

[FIGURE 3 OMITTED]

EXPERIMENTAL

Materials

A carboxylic functional polyester resin (supplied by DSM Coating Resins), with properties according to Table 1, was used to produce the different coatings employed in this study. As curing agent, [beta]-hydroxyalkylamide (Primid XL-552, in the form supplied from EMS-Chemie) was used as curing agent (Figure 3). The curing agent reacts with the carboxyl-functional polyesters, forming water during the condensation curing reaction.

All coatings were prepared with small amounts of benzoin (0.3 wt%) and flow agent (Resiflow PV88, 2 wt%) in order to reduce the defects caused by water evaporation during curing and to increase the flow in the coating during curing. (18)

The coatings were produced as unfilled clearcoats and as filler-reinforced coatings. The fillers were added to the coatings with three different pigment volume concentrations (PVC): 5, 15, and 25 vol%, respectively. Two types of fillers were used: Wollastonite and Barite. The Wollastonite (CaSi[O.sub.3], Casiflux A38 from Ankerpoort) is a needle-shaped calcium silicate with a maximum size of 38 [micro]m and a density of 2.9 g/[cm.sup.3]. The Barite is a crystalline barium sulphate (BaS[O.sub.4], Portaryte B 15 also from Ankerpoort) with particles of a more rounded shape with a maximum size of 25 [micro]m and a density of 4.4 g/[cm.sup.3] (Table 2).

The coatings were mixed with curing agent, fillers, and additives according to Table 3. After mixing, the blends were extruded, crushed, grinded, and sieved at Teknos OY, Finland, as in conventional powder production.

Sample Preparation

All coatings that were to be tested on a substrate were applied to phosphatized steel plates. The phosphatization was performed in a nine-step Bonder 28 Zn/Ni-phosphatizing process with Zirconium fluoride as passivation. The phosphatization was used for good adhesion between substrate and coating.

Free-standing, unsupported films were prepared by applying the powder coatings to aluminized steel with a thin layer of poly(vinylidene difluoride) (PVDF) on the surface. The surface tension of the PVDF was low enough to facilitate poor adhesion between the experimental coating and the PVDF film and to ease the removal of the cured experimental coating, yet the surface tension was high enough to avoid crawling of the coating during flow. (6)

The coatings were applied with a friction gun and cured at 190[degrees]C (object temperature) for 15 min.

The thickness of the cured coatings when applied to a steel substrate was measured with magnetic induction (Minitest4100). The thickness of the free-standing films was measured with a Mitutoyo micrometer device.

All coatings were subjected to conventional coating tests such as stone chip resistance, cross-cut test (ISO 2409), Erichsen impact test (ISO 6272), and Erichsen cupping test (ISO 1520). All the coatings exhibited similar performance, and no variations in their properties could be verified by these methods.

Thermal Properties

DIFFERENTIAL SCANNING CALORIMETRY (DSC): The thermal properties of the coatings were measured with a Mettler Toledo DSC 820. Heating and cooling rate was set at 10[degrees]C/min in all tests, and all samples were heated to 110[degrees]C and cooled down to 25[degrees]C (-10[degrees]C/min) before measurement, in order to avoid variations in thermal properties due to physical aging or other thermal prehistories. Approximately 8-12 mg of the material was used in each DSC measurement.

[FIGURE 4 OMITTED]

THERMOMECHANICAL ANALYSIS (TMA): The softening of the coating during indentation and compression was studied with a Mettler Toledo TMA 40. A quartz probe with a cylindrical contact area (diameter of 1.1 mm) was used to create an indentation in the coatings. The maximum load was 0.5 N, giving a contact stress of approximately 0.5 MPa.

Tensile Properties

The tensile properties of the coatings were measured on free-standing films with a TA Instruments Q800. Stress-strain curves were performed with constant stress rate (3 N/min) at three temperatures, 25[degrees], 50[degrees], and 70[degrees]C. The samples were 10-20 mm long and 2-5 mm wide with a thickness of 60-100 [micro]m. The length-to-width ratio was approximately 5:1 in all samples.

Equipment for Compression in a Clamping Force Joint

An experimental set-up was developed in order to study the creep and relaxation of coatings under load, simulating the set-up of a clamping force joint constructed of coated steel objects, screw and nut. In the setup developed, the screw was a Hookean material with a linear relation between applied tensile force and deformation in the range of the experimental force. During mounting (Figure 4), a force was applied to the screw causing a tensile deformation in the screw, [DELTA][l.sub.screw], (force-deformation line O-A-E). At the same time, the object was subjected to a compressive stress and deformed, [DELTA][l.sub.object], giving the force-deformation line O-G. The slope of the force-deformation curve indicates the stiffness of the part (Figure 4 shows the lower stiffness of the screw due to its smaller load-bearing area compared to the objects). The tensile force in the screw and the compressive force in the object reached the desired clamping force, [F.sub.p]. Line O-G was transported parallel to intersect with line O-A-E at point A, forming line B-A.

[FIGURE 5 OMITTED]

When the object (in this case, the coated steel object) settled during use, Z, the deformation of the object was increased ([DELTA][l.sub.object] + Z) and the object curve, B-A, shifted to the left (to the broken line, D-C). This caused a decrease in the clamping force from [F.sub.p] to [F'.sub.p]. The strain in the screw decreased ([DELTA][l.sub.screw] - Z) when the clamping force decreased, and the loss of clamping force in the screw gave an estimate of the deformation of the object (Figure 1B) in the joint. There is a direct relation between loss of clamping force and deformation in coating/object since the screw is linear elastic. The loss of clamping force can therefore be used as the value measured in the set-up for quantifying the deformation of the coating in the clamping force joint.

The coatings were applied to 5-mm thick phosphatized steel rulers. The coated objects were clamped with screws and nuts between two thick washers with specific contact areas. The washers were cylindrical with an inner diameter of 15.0 mm and an outer diameter of either 37.0 mm or 28.0 mm, giving a compressive area of 900 [mm.sup.2] and 440 [mm.sup.2], respectively. The schematic set-up of the equipment is shown in Figure 5.

The screws used in the study were 10.9 M14 with a maximum clamping length of 100 mm. The width of each screw was decreased below the head in order to make it possible to mount strain gauges on it. Strain gauges, type EA-06-062TJ, were glued on it as shown in Figure 6. Two gauges were placed at each side of the screw, and the average value of the strain from the four gauges was used as the measured strain. The strain gauges formed a full Wheatstone bridge, which, due to the set-up, was independent of changes in temperature and bending of the screw. The strain gauge set-up in each screw was calibrated to give a linear correlation between increases in resistance and applied tensile stress on the screw.

The screw was stretched by applying a torque on the nut, giving a specific clamping force. The total clamping length was 80 mm and all samples were clamped to 54 kN, which corresponds to a compressive mean stress of 60 or 120 MPa, depending on the geometry of the washers. In connection with each test, a dummy test with an un-coated steel ruler was run in order to verify that the loss of clamping force was not due to measuring artefacts such as deformations of the steel substrate or of the screw threads.

[FIGURE 6 OMITTED]

[FIGURE 7 OMITTED]

The loss of clamping force during the tests was, therefore, solely related to creep or relaxation of the coating, depending on time and temperature. The temperature was ramped from ambient temperature to 100[degrees]C, a temperature well above the [T.sub.g] of the coating ([approximately equal to][T.sub.g] + 30[degrees]C). The computer was monitoring the force of the screw and the temperature of the coating as a function of oven temperature and time. The thermo element measuring the coating temperature was mounted in a dummy between the washer and the coated steel ruler.

Tests were also performed with two coated steel objects clamped between the washers in order to simulate a normal clamping force joint consisting of two coated steel parts clamped with screw and nut. The clamping length in this experiment was increased to 85 mm, due to the thickness of the extra steel object, and the amount of powder coating in the set-up was increased by a factor of two.

The surfaces subjected to compressive loads were measured regarding surface topography before and after the clamping tests. The measurements were performed on a Perthometer PGK 120 from Mahr GmbH. The equipment was calibrated with standardized surfaces with a maximum deviation of 5% within the standard.

Equipment for Simulated Mounting-Friction And Initial Settlement in the Joint

The friction between coating and nut during mounting was measured in an Erichsen screw testing machine, Model 541, with software TesTwinner 920. The coatings were applied to 10-mm thick phosphatized steel rulers. The friction was studied by applying a torque to the screw/nut and measuring the build-up of the clamping force. The maximum torque was set at 135 Nm and was supposed to give a clamping force between 51 and 60 kN for a friction between 0.10 and 0.14. The rotational speed of the nut during the assembly was 10 rpm. The total friction was measured during mounting and divided into two types, friction between the screw and nut threads and friction between nut and coating. Standardized nuts and screws (M14) with specified friction were used in order to reduce the variations in friction between screws and nuts.

[FIGURE 8 OMITTED]

The initial settlement of the coating after mounting was studied by removing the applied torque and measuring the reduction in clamping force during the first 60 sec after mounting. Ten measurements of friction and settlement were performed on each coating, all of them under ambient conditions.

RESULTS AND DISCUSSION

Compression of Coatings in a Clamping Force Joint

The results from the compression of coatings in a clamping force joint reveal a clear influence of the coating thickness on the properties of the joint (Figure 7). Increasing thickness results in an increased loss of clamping force, which could be expected since the thicker the coating the more material is deformed in the clamped joint. However, the results in Figure 7 also indicate a reduced thermal stability of the coating when the thickness is increased. There is a shift towards lower temperatures at the onset of deformation, which was not expected. The loss of clamping force at lower temperatures can be correlated to the increase in sensitivity to defects when the coating thickness is increased.

The [beta]-hydroxyalkylamide curing polyesters cure by condensation reaction, forming water as a decomposition product. The water must evaporate through the coating, and the thicker the coating the longer the diffusion path of the water molecules. Flow agents and benzoin are usually added to the coating in order to promote the evaporation of water. The defects are called pin-holes, or small craters, and have historically been discussed regarding surface finish and gloss. However, the defects do not only occur in the surface but may also occur in the polymer matrix of the coating. A thick coating runs a higher risk of forming defects and, therefore, has a lower cohesive strength (Figure 9).

[FIGURE 9 OMITTED]

[FIGURE 10 OMITTED]

[FIGURE 11 OMITTED]

[FIGURE 12 OMITTED]

The coating is stabilized by the substrate, which reduces the mobility of the coating during creep under compressive load. The stabilizing effect of the substrate is limited by the geometry of the sample and the thickness of the coating. The limiting thickness of the powder coatings used in this study was approximately 150 [micro]m, as can be seen in Figure 10. Regardless of thickness, the coatings showed similar behavior below this limit, but above 150 [micro]m there was a significant drop in stability. This effect could also be observed when the number of coated objects in the joint was increased, but the thickness of each individual layer remained similar as in the one layer test. The clamping force remaining after two hours at 100[degrees]C in a joint consisting of two coated objects is shown in Figure 11 as a function of total coating thickness. The objects were coated on both sides, and the clamping force of the joint remained stable over a long stretch of increasing coating thickness up to 600 [micro]m. There is obviously a limiting coating thickness of approximately 150 [micro]m for each layer (four layers at 150 [micro]m equals 600 [micro]m in total) in the coatings used.

However, when the added coating thickness exceeded 600 [micro]m, the drop in clamping force was pronounced. Mounting coated objects with a total coating thickness of 600 [micro]m is made possible by the fact that the stresses on the coatings between the clamped parts is lower compared to the stresses under the washers, since the area of the clamped objects is larger than the area under the washers (see the schematic sketch in Figure 11). The loads were uniaxially compressed in these tests and the flow of the coating was restricted to the filling of voids and flow around the edges of the washers. The coatings were subjected to shearing loads at the flow around the edges but were hindered from flow due to the adhesion between coating and substrate, and this had a stabilizing effect on the set-up. The possibility of creep and deformation of the polymer would have been more pronounced if the adhesion between coating and substrate had been poor.

Surface roughness also affected the properties of the coatings, when they were subjected to compressive loads. The true stresses on the rough surfaces were much higher than the mean stress (60-120 MPa) on the object during the build-up of compressive load. The stress concentrations on the surface caused plasticization on the rough parts, and the contact area increased as the compressive load increased. The coated samples showed some orange peel as in most powder coatings, but the orange peel had only a minute influence on the deformation of the coating. However, the coatings prepared with Wollastonite fillers were rough, and there seemed to be a restricted flow of the coating during the levelling and curing. The mean surface roughness of the coatings was measured before and after compression in the clamping force joint. The [R.sub.z] value (ISO 4287) of the Wollastonite-filled coatings decreased from approximately 25.3 [micro]m to 3.6 [micro]m. This can be compared with the change of the unfilled clearcoats from 1.8 [micro]m to 1.7 [micro]m, due to compression and the change of the barium-sulphate-filled coatings from 3.2 [micro]m to 1.7 [micro]m. The compression of the surface roughness is a continuous process, starting directly after mounting and continuing during the ramp-up of the temperature (Figure 8). The plasticization of the polymer is a viscous process and, hence, dependent on stress level, temperature, and also time. The rate of plasticization increases during the ramp-up of the temperature and there is a tendency of shifting the onset of the drop in clamping force to a lower temperature and to a higher deformation rate when the surface roughness is increased (Figure 8). Note the difference in scale, however, on the ordinate in Figures 7 and 8. The loss of clamping force due to surface roughness is much less than the loss due to increased coating thickness. The increased void content in the thicker coatings may, therefore, be considered more significant for the loss of clamping force than the surface roughness.

The negative effect of the filler, in the form of increased plasticization due to increased roughness, is mainly caused by the Wollastonite filler. A further development of the coating composition can most likely reduce the roughness and improve the properties of the coating. The positive effect of the filler in the clamping force joint is visualized in Figure 12. The coatings filled with the round-shaped barium-sulphate filler were compared with the unfilled analogue, the latter showing a much higher creep and relaxation compared to the filled coatings. The filler obviously has a reinforcing effect when the coating is subjected to compressive loads at elevated temperatures. Coatings with filler reach a plateau at elevated temperature, where there is no further loss of clamping force and, hence, no further deformation of the coating. There are various possible explanations for the phenomena. For example, the increased amount of voids in the clearcoat is due to the higher amount of curing agent per [micro]m coating thickness. Other possible explanations are the stabilization brought about by the interactions between filler and resin or that the resin is deforming until the load is only being supported by the fillers and the fillers form a "heap of gravel." The behavior of coatings under compression is most likely a combination of the explanations mentioned, showing that coatings should be visualized as thin composites reinforced by the inorganic fillers and pigments. The fillers reinforce the polymer matrix and the flow of material is restricted by the fillers as well as by the adhesion between coating and substrate.

[FIGURE 13 OMITTED]

Simulated Mounting

All samples showed a relatively consistent friction, regardless of the structure of the coating. The mean value of friction between nut and coating remained at approximately 0.14, even though the coating thickness and filler content were varied from 80 to 350 [micro]m, and 0 to 25 vol%, respectively (Figure 13). What to a greater extent affects the friction is the shape of the sample and whether parts of the material undergo deformation due to shearing under the nut. Loose particles from the material will increase the friction.

Variations in the loss of clamping force directly after mounting and after removal of the mounting torque are not caused by the type of powder coatings used in this study. As shown in Figure 14, the coating thickness has an effect on the initial loss of clamping force. The loss of clamping force is dependent on the relaxation of compressed material, and the greater the amount of material, the greater the relaxation. The coating is subjected to high shearing forces in combination with compressive forces during the mounting. The area under the nut is approximately 130 [mm.sup.2], and a clamping force of 54 kN would result in a compressive stress on the coating of over 400 MPa. Such high compressive forces in combination with the shearing will cause plastic deformation of the coating and possibly also piling up of material around the borders of the nut. The amount of material between nut and coated object is, therefore, lower than before mounting, and the coating is more densely packed. Therefore, the relaxation is less than what could be expected. The independence of the amount of filler is most likely due to the same effect.

[FIGURE 14 OMITTED]

[FIGURE 15 OMITTED]

Thermal Analysis 

DIFFERENTIAL SCANNING CALORIMETRY (DSC): DSC reveals no effect from the type and amount of filler on the [T.sub.g] of the coatings when measured after heating and cooling controlled in order to reduce the effect of thermal prehistory and moisture. The [T.sub.g] was 69[degrees]C ([+ or -]0.8[degrees]C) regardless of the coating composition. However, variations in thermal properties of the coatings were observed when measuring [T.sub.g] during initial heating (without controlling prehistory in the DSC) after storage at ambient temperature in 50% RH for one week. The initial heating showed a shift in [T.sub.g] towards a lower temperature when the coating thickness was increased. The reduction in [T.sub.g] was 2-4[degrees]C when the coating thickness was increased from 70 [micro]m to 200 [micro]m, and was pronounced when the amount of filler was decreased, indicating that the changes in thermal properties were mainly due to changes in the polymer film. This may partially explain the reduction in onset of the loss in clamping force due to increased coating thickness as shown in Figure 7. The actual [T.sub.g] of the coating was not reduced with increasing coating thickness, since the [T.sub.g] after preheating was not affected by either coating thickness or filler content. The [T.sub.g] was not increased after extra curing either (20 min at 200[degrees]C), which indicates that the degree of curing was not affected by the coating thickness. However, the physical structure of the coating was affected by the coating thickness. The internal stresses in the coating increased with increasing coating thickness, (12) and the amount of water formed during curing and the diffusion path also increased with increasing coating thickness, producing more defects due to bubbles in the coating. These effects together created a weaker and less dense coating with a higher amount of voids. A less dense coating may result in an increased sensitivity to water and increased water absorption during storage after curing. Therefore, the decrease in initial [T.sub.g] with increasing coating thickness may most likely be due to increased absorption of water, acting as a plasticizer in the polymer matrix.

However, the decrease in the start temperature of the loss of clamping force shown in Figure 7 is more than the 2-4[degrees]C drop in initial [T.sub.g], and must therefore also be due to changes in the mechanical properties and not only to thermal properties.

THERMOMECHANICAL ANALYSIS (TMA): The possibility of changes in mechanical properties due to coating thickness was studied with TMA. However, the TMA results showed no additional effect on the [T.sub.g] as compared to the results from thermal properties in the DSC. Thus, no clear relation to the results in a clamping force joint shown in Figure 7 could be observed. However, the loads applied in TMA tests were several magnitudes lower than in a clamping force joint and the changes in performance in the joint were due to viscoelastic or viscoplastic behavior and yield at high stresses, and therefore would not occur at the low stresses in the TMA.

Tensile Tests

The results from the clamping force joint test in Figure 12 show that the fillers had a reinforcing effect at elevated temperatures, and this effect could be verified in tensile tests using a TA-Instrument Q800. The tensile modulus was increased with increasing PVC and the reinforcement from the fillers was evident also when the temperature was increased towards the [T.sub.g] of the coating (Table 4).

The coating became more brittle with the increase of filler content. The values for strain at break (Figure 15) were especially lowered, but the increased brittleness was also evident, since the rupture in the highly filled coatings occurred in most cases at the clamp.

The sensitivity to coating defects in measuring mechanical properties was also evident in the tensile tests. The amount of voids in the coating film and the roughness of the surface increased with increasing coating thickness and, therefore, affected the tensile tests. The stresses in the coating are estimated by dividing the load by the measured area of the cross section of the sample. However, the value given in measuring the coating thickness with magnetic induction or with a micrometer device is the maximum thickness, which is not the geometrically interesting value for the tensile tests. The area that has decreased due to voids and roughness cannot be measured, and the local stresses are thus higher than calculated. The results from the thick coatings varied significantly and could therefore not be used to draw quantitative conclusions from the tensile tests. The coating filled with 25 vol% Wollastonite had a rougher surface than the Barite-filled coating, and the tensile modulus of the former may therefore be higher than shown in Table 4. The needle-shaped Wollastonite had a higher reinforcing effect compared to the round/square-shaped Barite at 70[degrees]C, which may be due to the higher aspect ratio of the Wollastonite. This effect is most likely more evident in tensile tests. It could not be seen in the clamping force tests where the load was compressive.

CONCLUSIONS

Coating thickness is the most important factor that has to be controlled when using powder-coated steel parts in clamping force joints. Coatings that are too thick will not only result in the deformation of too much material, but will also increase the rate of the deformation and reduce the temperature at which it starts under the compressive loads in the clamping force joint. There are indications of a limit in thickness for the polyester powder coatings. The properties of the coating in the joint are more or less unaffected by the coating thickness up to approximately 150 [micro]m, but at higher thicknesses the stability of the coating is severely reduced. The change in behavior is due to a reduction in the stabilizing effect of the rigid substrate and to an increase in the amount of coating defects. Carboxyl-functional polyester powder coatings cured with [beta]-hydroxyalkylamide form water during curing. Coatings that are too thick hinder the evaporation of this water, and voids and gas bubbles are formed in the coating film. The voids and bubbles induce stress concentrations in the film which cause the coating to deform more rapidly. The voids also increase the diffusivity of the water in the coating, and absorbed water will act as a plasticizer lowering the [T.sub.g] of the coating. The lowering of [T.sub.g] in combination with stress concentrations at the voids reduces the temperature at which the material starts to deform.

Fillers have a reinforcing effect on coatings subjected to compressive loads. The creep and flow of the thermoset polymer matrix is restricted due to the interaction with the inorganic filler. The restriction of flow is especially evident at temperatures above the glass transition of the coating where an unfilled clearcoat deforms continuously under compressive loads, whereas a filled coating reaches a stable plateau at which it does not deform further.

Table 1 -- Properties of Polyester Resin

           Acid Value  [T.sub.g] (a)  Mn (b)   Mw (b)
           (mg KOH/g)  ([degrees]C)   (g/mol)  (g/mol)

Polyester  53.8        56.9           3140     14100

(a) Determined by DSC
(b) Determined by SEC

Table 2 -- Properties of Fillers

                                                            Particle
                 Density         Hardness       Diameter    Size
Filler           (g/[cm.sup.3])  (Moh's Scale)  ([micro]m)  ([micro]m)

Wollastonite     2.9             5              10          25-38
(CaSi[O.sub.3])                                             (sharp)
Barite           4.4             3.5            10-25       10-25
(BaS[O.sub.3])                                              (round)

Table 3 -- Variations in the Content of Coatings

                     Coating Recipes Before Mixing (g)
                           5%            15%           25%
Content         Clearcoat  BaS[O.sub.4]  BaS[O.sub.4]  BaS[O.sub.4]

Curing agent
Primid XL-552     75         75            75            75

Resin
Polyester       1000       1000          1000          1000

Filler
Portaryte B 15              192           645          1218
Casiflux A38                                            127

Additive
Benzoin            3          3             3             3
Resiflow PV88     20         20            20            20

                     Coating Recipes Before Mixing (g)
                   5%            15%           25%
Content         Wollastonite  Wollastonite  Wollastonite

Curing agent
Primid XL-552     75             75            75

Resin
Polyester       1000           1000          1000

Filler
Portaryte B 15
Casiflux A38     425            803

Additive
Benzoin            3              3             3
Resiflow PV88     20             20            20

Table 4 -- Young's Modulus of Unfilled and Filled Coatings (Coating
Thickness 70-85 [micro]m of all Samples)

                  Young's Modulus   Young's Modulus   Young's Modulus
                  at 25[degrees]C   at 50[degrees]C   at 70[degrees]C
Coating           (GPa)             (GPa)             (GPa)

 0%               1.9 [+ or -] 0.2  1.6 [+ or -] 0.1  0.5 [+ or -] 0.2
25% Barite        4.0 [+ or -] 0.2  2.7 [+ or -] 0.3  0.7 [+ or -] 0.1
25% Wollastonite  3.3 [+ or -] 0.2  2.5 [+ or -] 0.1  1.2 [+ or -] 0.2

ACKNOWLEDGMENTS

Hans Stenbrink at Teknos OY is acknowledged for supplying the coatings used in the study. Financial support for this work was provided by Scania CV AB and by a grant from the Swedish Agency for Innovation Systems (VINNOVA), project 2001-05573. Elodie Giroud, MSc, is thanked for carrying out part of the experiments in the clamping force tests.

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Daniel Stahlberg and Mats Johansson ([dagger]) -- Royal Institute of Technology*

* Fibre & Polymer Technology, SE-100 44 Stockholm, Sweden.

([dagger]) Author to whom correspondence should be addressed. Email: [e-mail]; voice: +46.8.790.9287; fax: [phone].